Facilitation and inhibition of phrenic motoneuronal activities by lung inflation

The purpose was to evaluate the facilitatory influence of pulmonary inflations on phrenic activity. In decerebrate cats, activities of phrenic motoneurons and nerve were recorded during ventilatory cycles in which the lungs were inflated to different levels and inflations were withheld. Motoneuronal activities were divided into 'early' and 'late' populations depending on their onset of activity. In normocapnia, facilitation was manifested by an increase in the rate of rise of phrenic activity. Facilitation increased with an increased level of inflations and fell when inflations were withheld. This facilitation was largely due to an increased rate of change and earlier onset of late motoneuronal activities. These variables for early motoneuronal activities were little altered by changes in inflations. Peak discharge frequencies of both early and late motoneurons increased during noninflation cycles. Facilitation was still evident during hypercapnia and in anesthetized animals; however, under these conditions, the earlier onset of late motoneuronal activities was no longer observed. We conclude that facilitation by pulmonary stretch receptor discharge is a constant determinant of phrenic neuronal and neural activities.